The measurement and control of humidity has been a very difficult and relatively expensive function. While there are many materials that are sensitive to humidity, the reliable and repeatable application of these elements for control of humidity has been found to be difficult. Most simple and inexpensive humidity responsive devices and materials yield a humidity measurement which is unreliable and which generally is not overly accurate. Simple acting elements to sense humidity have been used for many years to provide indicators for the percent of relative humidity. More accurate humidity sensing and/or control has relied on relatively expensive humidity sensing elements and electronics. Since most humidity indicating systems are temperature responsive, some type of temperature compensating has had to be applied to the typical humidity control in order to obtain any reasonable level of accuracy.
As a result of the difficulty of measuring and controlling humidity accurately, there are a large number of humidity sensitive indicators and control systems, but cost and accuracy problems plague most of these units.